This invention relates to encryption systems and more particularly to an encryption system for use with automated teller machines (ATM) and cash dispensers (CD). Such machines are typically accessed by means of a card issued by the customer's bank. Within the past twelve years the number of these machines has grown from a few scattered units to a worldwide total of almost 50,000 units. In many areas, groups of institutions have begun to cooperate in the establishment of local, regional, and national shared ATM/CD networks in order to extend the customer convenience represented by electronic fund transfer services beyond the local area.
Thus, a holder of a card issued by one financial institution (the "issuer") can transact business with the issuer through the ATM/CD of a different financial institution (the "acquirer"). This invention applies primarily to this type of transaction where security of one or more message elements must be provided throughout an interchange network communications system, as differentiated from security in a more restricted system not involving many institutions. It is also not limited to financial institutions.
Such networks typically rely on the use of some standardized identifying token which is presented by the user of such services. Such a token would be, for example, a user's plastic card with a magnetizable stripe on the card which is encoded with a particular set of data. It is necessary, however, to provide for the security and privacy of some of the data which is sent by such a user from the ATM/CD through intermediate stations, to the issuer's data processing center. These security provisions must meet needs for economical data transmission, preclude unauthorized access to critical customer related information, and provide a level of privacy that conforms to governmental regulations as they may be formulated. This level of protection must include the entry, transmission, storage, and verification procedures which are used by the various components of the interchange network.
Among the data elements of an ATM/CD message, the most critical requiring some form of protection are:
(a) the cardholder's personal identification number (PIN);
(b) the cardholder's primary account number (PAN);
(c) the cash advance or disbursement amount;
(d) the date and time of the transaction; and
(e) a transaction number (TN) which is varied from transaction to transaction.
There are crytographic techniques in existence which provide the means by which data elements such as these can be protected. Such a technique will be discussed in greater detail hereinafter. However, for the present purpose, it is sufficient to know that in the case of the PIN, for example, protection can be achieved by using a cryptographic process called "encryption" by which a PIN of 37 9725" might, for example, be converted temporarily to a disguised value of "B*7@" for transmission from an acquirer through an interchange network to the issuer. In this sense, the word "acquirer" would be the financial institution operating the ATM/CD, while the issuer would be, for example, the destination financial institution providing financial services to the user. At the destination of the message this disguised value would be converted to the original "9725" value by a reverse cryptographic process called "decryption" for further processing to verify the validity of the PIN.
For other critical message elements, such as the amount of the cash advance or disbursement, secrecy may not be required, only protection against alteration. Therefore, instead of encryption, a well-known cryptographic process called "message authentication" is used. This process uses each of the critical data elements in a sequential encryption-like computation that results in a "message authentication code, MAC" to be included, along with the protected data elements, in a message which is transmitted to the destination (intermediate or final).
At the destination, the MAC computation process is repeated on the same data elements. If any one or a combination of these elements has been modified while being transmitted through the interchange network, the resulting MAC would not, with reasonable probability, be identical to the MAC value received, and the message would be rejected because of probable fraud.
For an interchange encryption-decryption process to work, a standard for data protection must be used. In the United States, the American Bankers Association (ABA) Bank Card Standards Committee and the American National Standards Institute (ANSI) have adopted the standard published by the National Bureau of Standards (NBS) of the U.S. Government as the basis for this type of security. A brief description of the NBS concept is presented here for reference. The elements of the concept include an algorithm called the data encryption standard (DES) algorithm and a secret key. The DES is a set of complex mathematical transformations that has been published and is known to everyone, including potential adversaries. The secret key consists of 64-bits of data, known only to the system participants, that make the use of the published algorithm unique and secure.
The DES has the property of "reversibility"; i.e. the DES and the secret key can be used to "encrypt" the input data for protection. They can also be used to "decrypt" or reverse the protected data back to its original form with the same key that was used for the encryption process. A secret 64-bit key establishes security of the encryption system. The input can be any desired 64-bit combination of data. On command, the DES system subjects the input to sixteen complex transformations and presents the 64 resultant "ciphertext" bits at the output register. By ciphertext is meant that the text would be enciphered and not intelligible when reading or computer-based analysis were attempted.
As long as potential adversaries are prevented from learning the key, data for the typical cash advance or disbursement can generally be assumed to be secure. There are no known methods of attacking the system analytically. For a known input/output pair, solving for the key through "exhaustive" sequential testing of all possible (approximately) 72,000,000,000,000,000 values of the key does not appear to be practical within the near future. Use of two or more sequential encryption processes with corresponding different keys would require millions of years of processing by the fastest computers for exhaustive testing, thereby making any such attack completely infeasible.
As described above, a DES key consists of 64 bits which can be interpreted as 16 hexadecimal characters (0-9, and A-F). The security of any system based on DES processing is dependent upon the integrity of key generation and distribution as well as upon the human-related management and operational procedures established for the system. While there are a number of such keys to be used in this type of system, the two types of such keys which have relevance to the present discussion are a data-encryption or session key, and a key-encryption or master key.
A session key is a one-time key only used for the life of one transaction. In some manner, the session key must be sent from the sender to the receiver and the sending of the transaction must convey to the receiver the specific session key which was used for encrypting the transaction. No matter what method for informing the receiver is used, the session key must be protected during the transmittal process by encryption using a master key. Because the session key is used for only one transaction, the potential for compromise is reduced. The key-encryption key, or master key, however, is used for encrypting a session key being transmitted over normal data communication lines or stored in a host data processor. These master keys must be generated, distributed, and loaded under greater security control than that normally used for other types of keys. Because of the high level of security under which these keys are handled, master keys are typically used for longer periods of time that could extend into many months.
In an extensive network with a large plurality of acquirers and a large plurality of issuers, a switch station ("network switch") is used to route and co-ordinate the transaction requests and responses between the various acquirers and issuers. In such systems, it is simply not economically feasible to separately send session keys in special network messages. Also, the time requirements would be prohibitive.
The problems facing the operation of such ATM/CD networks are, then, to provide maximum-feasible security for the transaction data by encryption and decryption processes, and to securely, efficiently, and economically store, retrieve, and transmit the keys necessary to perform these processes.